U.S. patent application number 09/790194 was filed with the patent office on 2001-08-23 for charged-particle-beam optical systems and microlithography apparatus comprising a non-absorbing shaping aperture.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Yamamoto, Hajime.
Application Number | 20010016299 09/790194 |
Document ID | / |
Family ID | 18568342 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010016299 |
Kind Code |
A1 |
Yamamoto, Hajime |
August 23, 2001 |
Charged-particle-beam optical systems and microlithography
apparatus comprising a non-absorbing shaping aperture
Abstract
Charged-particle-beam (CPB) optical systems, and CPB
microlithography apparatus including CPB optical systems, are
disclosed that include a "shaping aperture" that absorbs a very low
percentage of incident charged particles and hence does not
experience excessive temperature increases due to bombardment by
and absorption of incident charged particles. Nevertheless, the
shaping apertures are effective for trimming and shaping a charged
particle beam to produce a downstream-propagating beam having a
desired transverse profile. The aperture opening in the shaping
aperture is defined in a conductive thin-film membrane. The
membrane thickness is configured to cause charged particles
incident on the membrane to experience scattering (e.g.,
forward-scattering). CPB optical systems including the shaping
aperture also include a "screening aperture" downstream of the
shaping aperture to block (absorb) scattered charged particles. The
screening aperture is made from a relatively thick conductive sheet
and is situated where the shaped beam forms a crossover downstream
of the shaping aperture.
Inventors: |
Yamamoto, Hajime; (Kawasaki,
JP) |
Correspondence
Address: |
KLARQUIST SPARKMAN
CAMPBELL LEIGH & WHINSTON, LLP
One World Trade Center, Suite 1600
121 S.W. Salmon Street
Portland
OR
97204-2988
US
|
Assignee: |
Nikon Corporation
|
Family ID: |
18568342 |
Appl. No.: |
09/790194 |
Filed: |
February 20, 2001 |
Current U.S.
Class: |
430/296 ;
250/492.1; 430/942 |
Current CPC
Class: |
G21K 1/02 20130101; B82Y
40/00 20130101; B82Y 10/00 20130101; H01J 2237/31776 20130101; H01J
2237/0453 20130101; H01J 37/3174 20130101 |
Class at
Publication: |
430/296 ;
430/942; 250/492.1 |
International
Class: |
G03C 005/00; A61N
005/00; G21G 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2000 |
JP |
2000-045811 |
Claims
What is claimed is:
1. A charged-particle-beam (CPB) optical system, comprising: a
shaping aperture situated and configured to receive a beam of
charged particles propagating along an optical axis from a CPB
source, the shaping aperture comprising a conductive thin-film
membrane defining an aperture opening that transmits at least a
portion of the beam incident on the shaping aperture, the thin-film
membrane scattering the charged particles of the beam incident on
the membrane without absorbing the incident charged particles, to
form a shaped beam propagating downstream of the shaping aperture;
and a screening aperture situated downstream of the shaping
aperture at a location at which the shaped beam forms a crossover,
the screening aperture comprising a conductive sheet defining an
aperture opening having a width dimension corresponding to a width
dimension of the crossover, the conductive sheet being sufficiently
thick in an optical-axis direction so as to absorb charged
particles incident on the sheet.
2. The system of claim 1, wherein: the charged particle beam is an
electron beam; and the thin-film membrane has a thickness that is
10 to 100 times a mean-free-path length of electrons in the
thin-film membrane.
3. The system of claim 2, wherein the conductive sheet of the
screening aperture is 500 to 1000 .mu.m thick.
4. The system of claim 1, further comprising a first condenser lens
and a second condenser lens situated at respective positions along
the optical axis, wherein: the first condenser lens is situated and
configured to converge the charged particle beam, propagating from
the CPB source, to form a "crossover" on the optical axis at a
principal plane of the second condenser lens; and the shaping
aperture is situated along the optical axis at the same position as
the second condenser lens.
5. The system of claim 4, wherein the shaping aperture is a
beam-trimming aperture configured to determine an aperture angle of
the charged particle beam emitted from the CPB source.
6. The system of claim 5, further comprising a profile-shaping
aperture situated downstream of the second condenser lens but
upstream of the screening aperture, the profile-shaping aperture
comprising a conductive thin-film membrane defining an aperture
opening that transmits at least a portion of the beam incident on
the profile-shaping aperture, the thin-film membrane scattering the
charged particles of the beam incident on the membrane without
absorbing the incident charged particles, to form a shaped beam
propagating downstream of the profile-shaping aperture.
7. The system of claim 6, wherein the profile-shaping aperture is
situated at an axial position at which an image of a CPB-emitting
surface of the CPB source is formed.
8. The system of claim 7, further comprising a third condenser lens
situated downstream of the profile-shaping aperture and upstream of
the screening aperture.
9. The system of claim 1, wherein the shaping aperture is a
beam-trimming aperture configured to determine an aperture angle of
the charged particle beam emitted from the CPB source.
10. The system of claim 9, wherein: the charged particle beam is an
electron beam; and the thin-film membrane has a thickness that is
10 to 100 times a mean-free-path length of electrons in the
thin-film membrane.
11. The system of claim 10, wherein the conductive sheet of the
screening aperture is 500 to 1000 .mu.m thick.
12. The system of claim 10, further comprising a first condenser
lens and a second condenser lens situated at respective positions
along the optical axis, wherein the first condenser lens is
situated and configured to converge the charged particle beam,
propagating from the CPB source, to form a "crossover" on the
optical axis at a principal plane of the second condenser lens.
13. The system of claim 12, further comprising a profile-shaping
aperture situated downstream of the second condenser lens but
upstream of the screening aperture, the profile-shaping aperture
comprising a conductive thin-film membrane defining an aperture
opening that transmits at least a portion of the beam incident on
the profile-shaping aperture, the thin-film membrane scattering the
charged particles of the beam incident on the membrane without
absorbing the incident charged particles, to form a shaped beam
propagating downstream of the profile-shaping aperture.
14. A charged-particle-beam (CPB) microlithography apparatus,
comprising: an illumination-optical system comprising the CPB
optical system of claim 1; and a projection-optical system situated
downstream of the illumination-optical system.
15. The CPB microlithography apparatus of claim 14, wherein: the
projection-optical system comprises first and second projection
lenses, and a contrast aperture situated axially at a beam
crossover between the first and second projection lenses; and the
contrast aperture comprises a conductive sheet defining an aperture
opening corresponding to the beam crossover, the conductive sheet
being sufficiently thick in an optical-axis direction so as to
absorb charged particles incident on the sheet.
16. The CPB microlithography apparatus of claim 14, wherein: the
illumination-optical system comprises a first condenser lens and a
second condenser lens situated at respective positions along the
optical axis, wherein the first condenser lens is situated and
configured to converge the charged particle beam, propagating from
the CPB source, to form a "crossover" on the optical axis at a
principal plane of the second condenser lens; and the shaping
aperture is situated along the optical axis at the same position as
the second condenser lens.
17. The CPB microlithography apparatus of claim 16, wherein the
shaping aperture is a beam-trimming aperture configured to
determine an aperture angle of the charged particle beam emitted
from the CPB source.
18. The CPB microlithography apparatus of claim 17, further
comprising a profile-shaping aperture situated downstream of the
second condenser lens but upstream of the screening aperture, the
profile-shaping aperture comprising a conductive thin-film membrane
defining an aperture opening that transmits at least a portion of
the beam incident on the profile-shaping aperture, the thin-film
membrane scattering the charged particles of the beam incident on
the membrane without absorbing the incident charged particles, to
form a shaped beam propagating downstream of the profile-shaping
aperture.
19. In a method for microlithographically exposing a pattern,
defined by a reticle, onto a sensitive substrate using a charged
particle beam propagating from a source through an
illumination-optical system to the reticle, and from the reticle
through a projection-optical system to a sensitive substrate, a
method for shaping the charged particle beam, comprising: providing
a shaping aperture and situating the shaping aperture to receive
the charged particle beam, the shaping aperture comprising a
thin-film membrane defining an aperture opening that transmits at
least a portion of the charged particle beam incident on the
shaping aperture, and the thin-film membrane being configured to
scatter the charged particles of the beam incident on the membrane
without absorbing the incident charged particles; passing the
charged particle beam through the aperture opening of the shaping
aperture to form a shaped beam propagating downstream of the
shaping aperture; providing a screening aperture and situating the
screening aperture downstream of the shaping aperture at a location
at which the shaped beam forms a crossover, the screening aperture
comprising a conductive sheet defining an aperture opening having a
width dimension corresponding to a width dimension of the
crossover, the conductive sheet being sufficiently thick in an
optical-axis direction so as to absorb charged particles incident
on the sheet; and passing the charged particle beam through the
aperture opening of the screening aperture.
20. A CPB microlithography method, comprising the beam-shaping
method of claim 19.
21. A device-manufacturing method, comprising the CPB
microlithography method of claim 20.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to charged-particle-beam (CPB)
"optical" systems as used, for example, in CPB microlithography
apparatus. Microlithography is a key technique used in the
manufacture of microelectronic devices such as semiconductor
integrated circuits, displays, and the like. More specifically, the
invention pertains to CPB optical systems comprising at least one
aperture serving to "trim" or shape the charged particle beam as
the beam passes through an opening defined by the aperture by
absorption of outlying particles of the beam.
BACKGROUND OF THE INVENTION
[0002] Conventional charged-particle-beam (CPB) optical systems
typically include at least one "shaping aperture" constructed of an
aperture plate defining an opening through which the charged
particle beam passes. The opening is sized such that, as the beam
passes through the opening, peripheral regions of the transverse
profile of the beam are clipped by respective edges of the opening.
Hence, shaping apertures generally are used, for example, for
trimming the beam, shaping the transverse profile of the beam, or
aligning the beam.
[0003] Conventionally, the aperture plate of a shaping aperture is
fabricated from a sheet of metal (e.g., molybdenum) having a
thickness sufficient to absorb the charged particles of the clipped
portions of the beam. This absorption causes heating of the
aperture plate. Excessive heating results in distortion and/or
damage to the aperture plate, which causes undesired changes in the
size and/or geometry of the opening. The heating also can extend to
neighboring structural components that can be deformed or damaged
by the heat. For example, elastomeric O-rings located near the
aperture can be deformed or damaged from heat.
[0004] The conventional approach to the problem of heating of the
shaping aperture is to cool the aperture plate actively, such as by
circulating a heat-exchange fluid through passages in the aperture
plate and surrounding structures. Unfortunately, this approach
results in substantial apparatus complexity and cost.
SUMMARY OF THE INVENTION
[0005] In view of the shortcomings of conventional apparatus as
summarized above, an object of the invention is to provide
charged-particle-beam (CPB) optical systems including a
beam-trimming and/or profile-shaping aperture (termed generally
herein a "shaping aperture") exhibiting substantially reduced
absorption of incident charged particles compared to conventional
systems. Another object is to provide CPB optical systems including
at least one shaping aperture that exhibits substantially less
heating during normal operation than conventional systems. Yet
another object is to provide CPB optical systems (including at
least one shaping aperture) having less complexity and lower cost,
compared to conventional systems, without compromising performance.
Yet another object is to provide CPB optical systems in which
temperature control of the shaping aperture(s) and neighboring
components is significantly easier to achieve, compared to
conventional systems.
[0006] To such ends, and according to a first object of the
invention, CPB optical systems are provided. An embodiment of such
a system comprises a shaping aperture and a screening aperture. The
shaping aperture is situated and configured to receive a beam of
charged particles propagating along an optical axis from a CPB
source. The shaping aperture comprises a conductive thin-film
membrane defining an aperture opening that transmits at least a
portion of the beam incident on the shaping aperture. The thin-film
membrane is configured to scatter the charged particles of the beam
incident on the membrane without absorbing the incident charged
particles, so as to form a shaped beam propagating downstream of
the shaping aperture. The screening aperture is situated downstream
of the shaping aperture at a location at which the shaped beam
forms a crossover. The screening aperture comprises a conductive
sheet defining an aperture opening having a width dimension
corresponding to a width dimension of the crossover. The conductive
sheet is sufficiently thick in an optical-axis direction so as to
absorb charged particles incident on the sheet.
[0007] By configuring the shaping aperture using a conductive
thin-film membrane, the current of absorbed charged particles of
the incident beam is limited to at most several percent of the
current of charged particles absorbed by a conventional shaping
aperture configured using a metal sheet. Hence, a shaping aperture
according to this embodiment experiences much less heating than a
conventional shaping aperture, thereby eliminating any need for an
active cooling system for the shaping aperture. Also, temperature
control of components near the shaping aperture is much simpler
than conventionally.
[0008] Charged particles that have been forward-scattered by the
thin-film membrane of the shaping aperture are blocked by the
screening aperture. Effective blocking is achieved by absorption of
the forward-scattered charged particles by the relatively thick
conductive sheet of the screening aperture (the conductive sheet
can be, for example, 500 to 1000 .mu.m thick), and by positioning
the screening aperture at a crossover. Thus, the screening aperture
prevents scattered charged particles from reaching the sensitive
substrate.
[0009] By way of example, the charged particle beam can be an
electron beam. In such an instance, the thin-film membrane
desirably has a thickness that is 10 to 100 times a mean-free-path
length of electrons in the thin-film membrane. With a thickness in
this range, most of the electrons in the beam incident on the
membrane pass through (with scattering) the membrane without being
absorbed by the membrane.
[0010] The CPB optical system can include a first condenser lens
and a second condenser lens situated at respective positions along
the optical axis. The first condenser lens desirably is situated
and configured to converge the charged particle beam, propagating
from the CPB source, to form a "crossover" on the optical axis at a
principal plane of the second condenser lens. The shaping aperture
desirably is situated along the optical axis at the same position
as the second condenser lens. The shaping aperture can be a
beam-trimming aperture configured to determine an aperture angle of
the charged particle beam emitted from the CPB source.
[0011] The system also can include a profile-shaping aperture
situated downstream of the second condenser lens but upstream of
the screening aperture. The profile-shaping aperture desirably
comprises a conductive thin-film membrane defining an aperture
opening that transmits at least a portion of the beam incident on
the profile-shaping aperture. The thin-film membrane scatters the
charged particles of the beam incident on the membrane without
absorbing the incident charged particles, so as to form a shaped
beam propagating downstream of the profile-shaping aperture. The
profile-shaping aperture can be situated at an axial position at
which an image of a CPB-emitting surface of the CPB source is
formed. This system also can include a third condenser lens
situated downstream of the profile-shaping aperture and upstream of
the screening aperture.
[0012] The shaping aperture can be a beam-trimming aperture
configured to determine an aperture angle of the charged particle
beam emitted from the CPB source. In such a configuration, if the
charged particle beam is an electron beam, the thin-film membrane
desirably has a thickness that is 10 to 100 times a mean-free-path
length of electrons in the thin-film membrane. CPB optical systems
in which the shaping aperture is a beam-trimming aperture also can
include a first condenser lens and a second condenser lens situated
at respective positions along the optical axis. The first condenser
lens is situated and configured to converge the charged particle
beam, propagating from the CPB source, to form a "crossover" on the
optical axis at a principal plane of the second condenser lens. The
system also can include a profile-shaping aperture, as described
above, situated downstream of the second condenser lens but
upstream of the screening aperture.
[0013] According to another aspect of the invention, CPB
microlithography apparatus are provided. An exemplary embodiment of
such an apparatus comprises an illumination-optical system
comprising a CPB optical system as summarized above. The apparatus
also includes a projection-optical system situated downstream of
the illumination-optical system. The projection-optical system
desirably comprises first and second projection lenses, and a
contrast aperture situated axially at a beam crossover between the
first and second projection lenses. The contrast aperture desirably
includes a conductive sheet that defines an aperture opening
corresponding to the beam crossover, the conductive sheet being
sufficiently thick in an optical-axis direction so as to absorb
charged particles incident on the sheet. The illumination-optical
system desirably comprises a first condenser lens and a second
condenser lens situated at respective positions along the optical
axis. The first condenser lens desirably is situated and configured
to converge the charged particle beam, propagating from the CPB
source, to form a "crossover" on the optical axis at a principal
plane of the second condenser lens. The shaping aperture desirably
is situated along the optical axis at the same position as the
second condenser lens. The shaping aperture can be a beam-trimming
aperture configured to determine an aperture angle of the charged
particle beam emitted from the CPB source. A profile-shaping
aperture can be included, situated downstream of the second
condenser lens but upstream of the screening aperture. If present,
the profile-shaping aperture desirably is configured to include a
conductive thin-film membrane defining an aperture opening that
transmits at least a portion of the beam incident on the
profile-shaping aperture. The thin-film membrane scatters the
charged particles of the beam incident on the membrane without
absorbing the incident charged particles, to form a shaped beam
propagating downstream of the profile-shaping aperture.
[0014] According to yet another aspect of the invention, methods
are provided for microlithographically exposing a pattern, defined
by a reticle, onto a sensitive substrate. The methods are performed
using a charged particle beam propagating from a source through an
illumination-optical system to the reticle, and from the reticle
through a projection-optical system to a sensitive substrate. In
this context, the methods are directed especially to shaping the
charged particle beam. In an embodiment of such a method, a shaping
aperture is provided and situated so as to receive the charged
particle beam. The shaping aperture comprises a thin-film membrane
defining an aperture opening that transmits at least a portion of
the charged particle beam incident on the shaping aperture. The
thin-film membrane is configured to scatter the charged particles
of the beam incident on the membrane without absorbing the incident
charged particles. The charged particle beam is passed through the
aperture opening of the shaping aperture to form a shaped beam
propagating downstream of the shaping aperture. A screening
aperture is provided and situated downstream of the shaping
aperture at a location at which the shaped beam forms a crossover.
The screening aperture comprises a conductive sheet defining an
aperture opening having a width dimension corresponding to a width
dimension of the crossover. The conductive sheet is sufficiently
thick in an optical-axis direction so as to absorb charged
particles incident on the sheet. The charged particle beam is
passed through the aperture opening of the screening aperture.
[0015] The invention also encompasses CPB microlithography methods
that comprise beam-shaping methods as summarized above, as well as
device-manufacturing methods that comprising CPB microlithography
methods as summarized above.
[0016] The foregoing and additional features and advantages of the
invention will be more readily apparent from the following detailed
description, which proceeds with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic elevational diagram of an exemplary
embodiment, according to the invention, of a charged-particle-beam
(CPB) optical system as used in a CPB microlithography apparatus,
wherein the CPB optical system includes both a beam-trimming
aperture and a profile-shaping aperture (with screening aperture)
according to the invention.
[0018] FIG. 2 is a schematic elevational diagram depicting
absorption of charged particles, scattered by impingement on an
upstream profile-shaping aperture, by a downstream screening
aperture.
[0019] FIGS. 3(a)-3(f) are schematic elevational diagrams showing
the results of respective steps in an exemplary method for
manufacturing a beam-shaping aperture according to the
invention.
DETAILED DESCRIPTION
[0020] This invention is described below in the context of a
representative embodiment. It will be understood, however, that the
described embodiment is not intended to be limiting in any way.
[0021] FIG. 1 is a schematic elevational diagram of an exemplary
embodiment, according to the invention, of a charged-particle-beam
(CPB) optical system configured for use in a CPB microlithography
apparatus. The FIG. - 1 apparatus is described in the context of
employing an electron beam as a representative charged particle
beam. It will be understood that the general principles of the FIG.
1 embodiment can be applied with equal facility to use of an
alternative charged particle beam such as an ion beam.
[0022] The FIG. 1 apparatus includes an electron gun 1 situated at
an extreme upstream end of the CPB optical system. The electron gun
1 emits an electron beam downward in the figure (i.e., the beam
emitted from the electron gun propagates in a downstream
direction). The electron beam as emitted from the electron gun 1
propagates along an optical axis A.
[0023] The FIG. 1 apparatus is configured for performing CPB
microlithography, and hence comprises an "illumination-optical
system" IOS and a "projection-optical system" POS. The
illumination-optical system IOS is situated between the electron
gun 1 and a "reticle" 19 that defines a pattern to be projected
microlithographically onto a "sensitive substrate" 27 (e.g.,
semiconductor wafer coated with a suitable "resist"). The
projection-optical system POS is situated between the reticle 19
and the substrate 27.
[0024] The illumination-optical system IOS comprises a first
condenser lens 3, a second condenser lens 5, a beam-trimming
aperture 7, a profile-shaping aperture 9, a third condenser lens
13, a "screening" aperture 15, and an illumination lens 17. The
projection-optical system POS comprises a first projection lens 21,
a contrast aperture 25, and a second projection lens 23.
[0025] Although this embodiment has both a beam-trimming aperture 7
and a profile-shaping aperture 9, in an alternative embodiment, the
beam-trimming aperture 7 could be omitted.
[0026] The electron beam emitted from the electron gun 1 is
converged by the first condenser lens 3 to form a "crossover" on
the optical axis at the principal plane of the second condenser
lens 5. The beam-trimming aperture 7 is situated at the same axial
position as the second condenser lens 5. The beam-trimming aperture
7 typically defines a circular opening that transmits the beam,
thereby determining the downstream aperture angle of the beam. The
beam-trimming aperture 7 can be made from a thin, electrically
conductive membrane of, e.g., silicon or the like.
[0027] The thickness of the membrane of the beam-trimming aperture
7 (i.e., the Z-dimension) generally is sufficient to cause charged
particles, incident on the membrane, to be scattered rather than
absorbed by the membrane. The membrane is also sufficiently thick
to have adequate mechanical strength to provide adequate service as
a beam-trimming aperture. For an incident beam of electrons, the
membrane thickness typically is within the range of 10 to 100 times
the length of the mean free path of electrons of the beam in the
membrane material. By way of example, the mean free path of
electrons in a silicon membrane is 150 nm for a 100 keV electron
beam. Under such conditions, the thickness of the silicon
beam-trimming aperture 7 can be about 2 .mu.m.
[0028] At 2 .mu.m thickness, the beam-trimming aperture 7 is
configured as a "membrane." Because the membrane transmits (with
scattering) incident charged particles, rather than absorbing the
particles, the membrane experiences very little heating from
impingement of incident charged particles. Hence, a beam-trimming
aperture 7 configured as a membrane is not subject to thermal
deformation.
[0029] The profile-shaping aperture 9 is disposed downstream of the
second condenser lens 5 at an axial position at which an image of
the electron-emission surface (cathode) of the electron gun 1 is
formed. The profile-shaping aperture 9 defines the transverse
profile of the electron-beam flux and determines the transverse
sectional area of the beam illuminating a region on the reticle 19.
The profile-shaping aperture 9 desirably is a membrane made of
silicon or the like, similar to the beam-trimming aperture 7. With
such a configuration, the profile-shaping aperture 9 does not
exhibit significant temperature increases from impingement of
incident charged particles. Hence, the profile-shaping aperture 9
is not subject to thermal deformation.
[0030] The third condenser lens 13 is disposed downstream of the
profile-shaping aperture 9, and the screening aperture 15 is
disposed at a crossover position downstream of the third condenser
lens 13. The screening aperture 15 is fabricated from a sheet of
metal, such as molybdenum or tantalum, desirably approximately 500
to 1000 .mu.m thick. The screening aperture 15 functions in
conjunction with the beam-trimming aperture 7 and/or
profile-shaping aperture 9, and serves to block (by absorption)
charged particles scattered by the beam-trimming aperture 7 and/or
the beam-shaping aperture 9. By relegating the task of
charged-particle absorption to the screening aperture 15 (which
does not have to define an aperture opening accurately that
otherwise would be deformed by heating), the beam-trimming and
profile-shaping apertures are relieved of having to be subject to
heating.
[0031] The illumination lens 17 is disposed downstream of the
screening aperture 15. The electron beam passing through the
illumination lens 17 forms an image of the profile-shaping aperture
9 on the reticle 19 situated downstream of the illumination lens
17.
[0032] The first and second projection lenses 21, 23, respectively,
are disposed downstream of the reticle 19. The contrast aperture 25
is disposed at a crossover location between the projection lenses
21, 23. The contrast aperture 25, similar to the screening aperture
15, is fabricated from a sheet of metal, such as molybdenum or
tantalum, desirably approximately 500 to 1000 .mu.m thick. The
contrast aperture 25 serves to block (by absorption) charged
particles scattered by the membrane portion of the reticle 19. An
image of the illuminated portion of the reticle 19 is formed, with
demagnification, on a corresponding region of the substrate 27 by
the first and second projection lenses 21, 23, respectively.
[0033] Although not described or shown herein, it will be
understood that each of the illumination-optical system IOS and
projection-optical system POS includes one or more deflectors and
corrective coils as used for beam scanning, beam-position
adjustment, and aberration control, for example.
[0034] FIG. 2 is a schematic elevational diagram depicting
absorption of charged particles, scattered by an upstream
profile-shaping aperture, by the downstream screening aperture. The
figure shows an enlargement of an area around the profile-shaping
aperture 9, third condenser lens 13, and screening aperture 15.
[0035] The charged particle beam transmitted through the opening
defined by the profile-shaping aperture 9 (representative
trajectories of transmitted charged particles are denoted by the
solid lines in the figure) are converged by the third condenser
lens 13 for passage through the opening defined by the screening
aperture 15. Meanwhile, charged particles impinging on the
profile-shaping aperture 9 are scattered as they pass through the
membrane of the profile-shaping aperture 9 (representative
trajectories of scattered charged particles are denoted by the
dashed lines in the figure). Most of the scattered charged
particles are not converged sufficiently by the third condenser
lens 13 for passage through the opening in the screening aperture
15. Rather, these scattered charged particles impinge on and are
absorbed by the aperture plate of the screening aperture 15. By
having the screening aperture 15, rather than the profile-shaping
aperture 9, perform the task of particle absorption, the
profile-shaping aperture 9 does not experience significant
absorption-based heating.
[0036] FIGS. 3(a)-3(f) are schematic elevational diagrams showing
the results of respective steps in an exemplary method for
manufacturing a beam-trimming or profile-shaping aperture according
to the invention. In a first step, a boron-doped oxide film 33D'
and a silicon membrane layer 32A are laminated on a major surface
(top surface in the figure) of a base substrate 31 (FIG. 3(a)). The
base substrate 31 desirably is made of silicon. A film of silicon
nitride (SiN) 35 is formed on the opposing major surface (bottom
surface in the figure) of the base substrate 31. The SiN film 35
serves as a mask during later etching of the base substrate 31 from
the bottom.
[0037] A resist film 36 is applied to the SiN film 35 (FIG. 3(b)).
A pattern for forming support struts 31A in the base substrate 31
is exposed into the resist film 36 and the resist is developed.
Using the developed resist film 36 as a mask, the SiN film 35 is
dry etched as shown in the figure to form a corresponding pattern
in the SiN film 35. The pattern defines the locations at which the
support struts 31A will be formed.
[0038] Next, the base substrate 31 is wet-etched from its bottom
surface ("backetched") according to the pattern in the SiN film 35
to form the girder-like support struts 31A and a bilayer membrane
consisting of the silicon membrane layer 32A and the boron-doped
oxide layer 33D'. The support struts 31A support the bilayer
membrane and form a membrane region 31B. The boron-doped oxide film
33D' is exposed in the membrane region 31B. The SiN film 35 and
resist film 36 remaining at the "lower" ends of the support struts
31 A (opposite from the boron-doped oxide film 33D') are stripped
away (FIG. 3(c)).
[0039] A film of resist 37 is applied to the membrane layer 32A. A
pattern defining the desired aperture opening (e.g., square) for
the membrane layer 32A is exposed into the resist film 37, and the
resist is developed (FIG. 3(d)).
[0040] The membrane layer 32A is dry-etched using the developed
resist film 37 as a mask, thereby forming the respective aperture
opening in the membrane layer 32A of the membrane region 31B.
Remaining resist film 37 is stripped away (FIG. 3(e)).
[0041] The boron-doped oxide film 33D' is removed in the membrane
region 31B using hydrofluoric acid, thereby completing fabrication
of the beam-shaping aperture (FIG. 3(f)).
[0042] By making the beam-trimming and profile-shaping apertures
from respective thin films according to the invention, absorption
of CPB beam current by these apertures is reduced to at most
several percent of the absorption that otherwise would be exhibited
by a beam-trimming or profile-shaping aperture made of a relatively
thick sheet of metal. As a result, the need to perform active
cooling of the beam-trimming or profile-shaping aperture(s) in a
CPB optical system according to the invention is eliminated,
resulting in simplification and cost reduction of the overall
system. Also, control of the temperature of components surrounding
the beam-trimming and profile-shaping apertures is simplified. In
addition, because charged particles forward-scattered from the
beam-trimming or profile-shaping aperture are blocked by a
downstream metal screening aperture, the scattered charged
particles are prevented from reaching the substrate, thereby
achieving maximal image contrast on the substrate.
[0043] Whereas the invention has been described in connection with
a representative embodiment, it will be understood that the
invention is not limited to that embodiment. On the contrary, the
invention is intended to encompass all modifications, alternatives,
and equivalents as may be included within the spirit and scope of
the invention, as defined by the appended claims.
* * * * *